1. Field of the Invention
This invention relates to tire treads, and more specifically relates to tire treads in which repeating design cycles or pitches are altered in order to control noise.
2. Description of Related Art
In the past, tire treads have been designed by the use of repeating design cycles or pitches extending circumferentially around the periphery of the tire. The circumferential length of the design cycles or pitches has been modulated in order to achieve objectives such as spreading the noise generated by the tire over a broad band of frequencies. In many such tires, multiple lengths of design cycles are utilized. Many tires have incorporated three discrete different sizes of design cycles or pitches that are placed around the peripheries of such tires. Typical ratios of lengths of the small, medium and large pitches used in such tires are shown in Table 1:
TABLE 1 ______________________________________ Ratios % Variation ______________________________________ 9-10-11 22.2 11-13-15 36.4 5-6-7 40.0 9-11-13 44.4 7-9-11 57.1 9-10-12 33.3 6-7-9 50.0 ______________________________________
Typical pitch ratios used in conventional three-pitch tires are listed under the column entitled "Ratios." The first entry under the column headed "Ratios," reading "9-10-11," indicates a tire in which the small, medium and large pitch lengths have the ratio 9-10-11. Opposite the pitch ratio is a number indicating the percent variation from smallest to largest pitch. The percent variation is obtained from subtracting the smallest term of the pitch ratio from the largest term of the pitch ratio, dividing the remainder by the smallest term of the pitch ratio and multiplying by 100. For the first entry in Table 1, the percent variation is defined as ((11-9)/9).times.100.
In this specification and claims, pitch ratios of three-pitch tires are expressed as three integers that cannot be reduced to three lesser integers by a common divisor.